Relaxation oscillator



sept. 3, 1940.

D. H. BLACK RELAXATION OSGILLATOR Filed Oct. 22, 1937 /NIVE/vroR o. H. amc/e Patented Sept. 3, 1940 UNITED stares RELAXATION OSCILLATOR Donald Harrison Black, London, England, assignor to International Standard Electric yCorporation, New York, N. Y.

Application October 22, 1937, Serial No. 170,398

' In Great Britain November i6, 1936 5 claims.

This invention relates to relaxation oscillation generators, that is arrangements in which an impulse or succession of impulses of desired wave form is obtained by the suitable charging and discharging of a condenser.

Such oscillators are frequently used to provide linear time bases for cathode ray oscillograph work and two such oscillators are normally employed when a cathode ray tube is used to receive television transmissions. One common method of producing the required relaxation or saw toothed oscillations is by means of gas discharge tubes, either of the diode or triode type. However, the use of such discharge tubes has a number of disadvantages and circuits using socalled hard valves have therefore been proposed. An object of the invention is to provide an improved method of employing a hard valve to obtain relaxation oscillations.

According to one feature of the invention a condenser in a relaxation oscillator is adapted to be discharged over a circuit comprising a secondary electron stream set up in a vacuum tube.

According to another feature of the invention a condenser, which is `adapted to be charged through an impedance is connected' across a primary discharge path of a tetrode or four electrode valve and the tetrode is connected so that when the condenser voltage is large a secondary discharge path is set up in the tetrode by secondary electrons reaching the positive electrode of the primary discharge path. The current in the secondary discharge path is utilised to provide a voltage for reducing the potential of the control grid of the tetrode so that the impedance of the discharge paths is rapidly reduced and the condenser rapidly discharged.

According to a further feature of the invention, a variable impedance suitable for the rapid discharge of a condenser in a relaxation oscillation generator comprises a screen-grid Valve subject to a control voltage adapted to vary the screen grid potential and thereby to vary the secondary emission from the anode and means for controlling the control-grid potential according to the anode current to increase and decrease the primary electron stream when the secondary emission is respectively increasing and decreasing.

Other features of the invention will become apparent in the following description of an embodiment of the invention which is illustrated in the accompanying drawing.

Referring to the drawing, a relaxation oscillator comprises a condenser C1 adapted to be charged over a resistance R1 from a direct cur- (Cl. Z50-36) rent supply source connected between the terminals HT+ and EIT- The' condenser is intermittently discharged by means of the valve V.

The valve V is of the tetrode screen-grid type and is such that its anode is capable of emitting secondary electrons to such an extent that the number of secondaries is greater than the number of primaries. The greater the ratio of the number of secondaries to the number of primaries the more rapid will be the discharge time, and therefore the anode surface is preferablytreated in such a manner as to raise this ratio to a conveniently high value.

For this purpose the anode may be formed or treated in any of the ways known in the construction of vacuum tubes known as electron multipliers and utilising the secondary emission from an electrode bombarded by primary electrons. For example, the anode may comprise a coating of caesium oxide on a surface of silver.

The anode potential is adjusted by means of a tapping on a resistance R4. and is set at some convenient value, say 100 volts. The rst, or control, grid potential is determined by -a tapping on the resistance R5. The resistances R5 and R4 are connected in series with a third resistance Ra between the supply terminals.

In operation the condenser C1 charges up through the resistance R1. No current passes in the valve V until the voltage across C1 reachesl .such a value that current begins to flow from the second, or screen, grid of the valve to the cathode. The potential at which this current starts is determined by the potential on the control grid. When electrons begin to arrive at the screen grid others will also arrive at the anode and these will eject secondary electrons from the anode which will be collected by the screen. Thus current will ilow from screen to anode, via the resistances R2 and R4 and the condenser C1. The current flowing in R2 causes a positive impulse to be applied to the first grid over the condenser Cz. As the rst grid becomes more positive the number of electrons leaving the cathode will increase, so increasing the discharge current from the condenser C, and at the same time causing increasing current to flow in R2 resulting in a still further increase in the positive potential applied to the control grid. The action thus becomes cumulative and the condenser C1 is discharged very rapidly. It should be noted that the condenser is discharged over two paths; (l) by the primary electrons flowing from cathode to screen, and (2) by the secondary electrons flowing from anode to screen. If the anode is so treated that it emits a copious supply of secondary electrons, this latter discharge path may be made very eective.

When the condenser C1 has discharged to 'such an extent that the primary electron current begins to fall oli, then the secondary electron current from anode to screen grid will also decrease. This will result in a negative pulse being transmitted to the control grid with a consequent further reduction in primary current and the process becomes cumulative, resulting in a rapid resetting of the valve to the non-conducting state.

Re is a decoupling resistance to prevent the impulses to the control grid from. owing to the cathode viaV R5. C3 and C4 are reservoir condensers of convenient value. v

The invention is not limited to the description embodiment above described and a number of variants will become apparent `based on the-same operating principle underlying the arrangement described. Synchronising impulses may be introduced into the control grid circuit in any known manner in order to initiate vthe discharge, as is the practice in most relaxation oscillators. 'I'he condenser C1 may be charged through a saturated diode or through a pentode valve operating on the vsaturation portion of its characteristicin order to obtain a more uniform charging rate. The device may also be followed by amplifying and phase inverting stages if required.

In the interests of brevity and precision the expressions relatively charging and relatively discharging are used hereinafter to denote opposite changes in the charge of a condenser without regard to whether'the absolute potential difference across the condenser is greater in the relatively lcharged or the relatively discharged state. 'Ihe expressions absolutely charging and absolutely discharging are used when it is intended to distinguish speciiically between changes of charge which vary the total potential diierence respectively away from and toward absolute zero.

2. Relaxation oscillation generator comprising a vacuum tube, a thermionic cathode, a control grid, a iirst positive electrode and a second positive electrode therein, a condenser connected between said cathode and said rst positive electrode, a circuit for relatively charging said condenser, means for polarising said second positive y electrode and said control grid so that primary and secondary electronslow to said rst positive electrode respectively from the cathode and saidv other positive electrode responsive to said condenser becoming charged and means for setting up a voltage proportional to the current to said second positive electrode and for utilising this voltage to modify the potenti-a1 of said control grid to increase the primary electron stream.

3. Relaxation oscillation generator comprising a direct current supply source, a condenser and an impedance connected in series across said source for the absolute charging of said condenser and an absolute discharging circuit for said condenser, said discharging circuit comprising a vacuum tube, having a thermionic cathode, a control grid, v an'a'node and a lfourth electrode therein, said condenser being connected between said cathode and said fourth electrode, means for applying between said cathode and said anode a voltage lessA than that of the supply source, means for biasing said control grid so that until said condenser has a substantial absolute charge substantially no current flows through said tube, a resistance in said `anode connection and a condenser connected between said anode and said control grid.

4. Relaxation oscillation generator comprising a vacuum tube having a thermionic cathode, a control grid, a third electrode and a fourth-electrode therein, meansvfor biasing said third electrode more positively than either said cathode or said control grid, ka, condenser, means for relatively charging said condenser, means for biasing said fourth velectrode to such a potential with respect lto said third electrode that secondary electrons flow from saidthird electrode to said fourth electrode and a circuit lfor relatively disg charging said condenser comprising a space conduction path extending from` said fourth electrode to said cathode and third electrode.

5. Relaxation oscillator according to claim A4, further comprising an impedance in series with said third electrode in said circuit, anda coupling condenser connected between `said Vthird electrode and said control grid.

DONALD AHARRISON BLACK.

inK0 

